Elastic current collector

ABSTRACT

The invention describes a current collector for electrochemical cells, consisting of a sandwich of compressible and resilient layers of metal wires, which imparts a predetermined mechanical load under a broad compression range.

[0001] The invention concerns a new current collector forelectrochemical cells, particularly useful for electrolytic cells, fuelcells or other types of cells separated into at least two compartments,wherein the separator is an ion exchange membrane or any other type ofsemi-permeable diaphragm characterised by a limited mechanicalresistance.

[0002] The current collector of the invention is useful for ensuring theelectrical continuity between two conductive surfaces separated by agap, which in the case of an electrochemical cell is typically exploitedfor feeding reactants, discharging products, circulating electrolytes orfor the thermo-regulation of fluids, or for a combination of two or moreof these functions.

[0003] The advantages obtained by this type of contact inside anelectrochemical cell, by means of deformable elastic elements, are wellknown to the experts of the art. Typical examples of deformable elasticcollector are metal foams and reticulated porous materials in general,as described for example in U.S. Pat. No. 4,657,650. Another example, oflarger industrial diffusion, is a sandwiched metal wire structure, asdescribed for example in U.S. Pat. No. 4,693,797. Deformable structuresof this type have the advantage of being capable of transmittingelectric current between two conductive surfaces partially compensatingfor their deviations from planarity, thanks to the different localcompression they may undergo. It is therefore advantageous resorting tothe same both in terms of mechanical as well as electricalcharacteristics, to improve the efficiency of the cells where they areutilised. In particular, it is evident that the efficiency ofremoval/transmission of electric current increases as the contactpressure exerted by the collector onto the conductive surfacesincreases. Further, said pressure must be preferably exerted making thecollector work under an elastic regimen, to compensate for possibledimensional variations due to expansions, vibrations or other phenomenawhich may vary the geometry of the system in a microscopic range. Thepressure exerted by the collector however should not exceed, in manypractical applications, a limit threshold to avoid mechanical damages.For example, it is known that in electrochemical cells where one or morecompartments use a semi-permeable diaphragm, for example an ion exchangemembrane, said separation elements have a very limited mechanicalresistance and resist to mechanical loads only below a certainthreshold. The problem of mechanical resistance however does not affectonly the separators, as it is known that electrochemical cells can beprovided with deformable electrodes, for example very thin metal meshes,or gas diffusion electrodes comprising carbon materials with limitedresilience, such as carbon paper or carbon cloth, which have a scarcelyreliable behaviour with loads exceeding for example 0.35-0.4 kg/cm².

[0004] The mattress described in U.S. Pat. No. 4,693,797 has found aremarkable industrial application in view of the fact that, if usedunder optimum conditions, it grants a suitable contact pressure(indicatively 0.2-0.35 kg/cm²) also when using ion exchange membranes orgas diffusion electrodes or both said components. The contact loadexerted by this type of collector is maintained by the deformationcaused by the squeezing when clamping the cell; that is the mattress isinserted in an uncompressed condition, thus with the maximum expansion,and then squeezed when clamping the cell, whereby its thickness isdecreased even by 50%. For example, a mattress with a thickness of 10 mmwhen uncompressed, may reach, under operation, a thickness of 4 or 5millimeters. In the case of the mattress of U.S. Pat. No. 4,693,797,this is a very critical factor, as the load curve under that compressionregimen is very sharp. That is, a slight error in the mechanicaltolerances is sufficient to impose to the mattress either a too smallload, insufficient to grant a good electrical contact, or too heavyload. In particular, if the sandwich of wires constituting the mattressis compressed until the wire bundles are totally collapsed, a furthercompression, even if slight, involves a very heavy mechanical load,which in many cases may even be localised into a very small region,wherein a possible mechanical deformation of the surfaces to becontacted may occur. In the case of a cell with elements not suitablefor bearing heavy mechanical loads, such a membranes or gas diffusionelectrodes, this may readily bring to a mechanical failure of thesecritical components. In addition to the primary cost of said components,also the costs connected to the shutdown of the cell for repairing andrelevant substitutions must be considered.

[0005] In this regard, a further inconvenience typical of the mattressof U.S. Pat. No. 4,693,797 is the positioning of the same in the cell,due to the fact that its periphery is deformable and thus its alignmentwith the other components of the cells as well as centering any gasketmay become a critical operation, to be carried out in any case manually.Cells equipped with this type of collector involve therefore additionalassembling and maintenance costs, as automated assembling would beundoubtedly risky.

[0006] It is an object of the present invention to provide for a currentcollector for use in electrochemical cells capable of overcoming theprior art drawbacks.

[0007] In particular, it is an object of the present invention toprovide a current collector capable of impressing a suitable load foruse in electrochemical cells, for example in cells provided withseparators such as diaphragms and membranes and/or gas diffusionelectrodes, under a broad compression range. Under another aspect, it isan object of the present invention to provide a current collector forelectrochemical cells which preferably permits an automated assembling.

[0008] Under another aspect, it is an object of the present invention toprovide an electrochemical cell, for example an electrolytic cell or afuel cell equipped with a current collector overcoming theinconveniences of the prior art.

[0009] The invention consists of a current collector obtained bysandwiching compressible and resilient layers, each one formed by anarrangement of metal wires. The main characteristic of the currentcollector is its capability of imparting a suitable load forapplications in electrochemical cells, indicatively comprised between0.15 and 0.40 kg/cm², in a wide range of compressions, equal to at least10% of the uncompressed thickness of the collector itself. In apreferred embodiment, said range is comprised between 20 and 60% of thecompression of the collector with respect to its uncompressed state.This means that, for example, a typical collector having a thickness,before compression, of 10 millimeters, may be compressed upon clampingof the cell with tolerances up to about more or less 1 millimeterwithout risking ruptures of delicate components or insufficient contact,which result cannot be obtained with the collectors of the prior art.Preferably, for a collector 10 mm thick in the uncompressed state, theideal operation thickness is comprised between 3 and 6 millimeters. Thecollector of the invention is preferably made by sandwiches of metalwires having a diameter indicatively comprised between 0.1 and 0.35millimeters; the thickness of the collector resulting from said sandwichis preferably comprised between 5 and 15 millimeters. The preferredmaterials for producing the collector of the invention are all themetallic materials, in particular valve metals, for example titanium andalloys thereof, for the anodic collector, and nickel or alloys thereoffor the cathodic collectors. Depending on the applications, thecollector of the invention my also be a bipolar collector, for exampleprovided with nickel layers facing the cathodic surface and titanium orother valve metal layers facing the anodic surface. Depending on theprocess conditions, it is possible to further coat the collector withmaterials providing protection against corrosion, for example thecathodic collectors with a silver coating and the anodic collectors witha coating of noble metals or alloys thereof or their oxides. Differentembodiments are possible for the collector of the invention, however, itis preferable that the most external layer be generally planar in orderto distribute the contact as uniformly as possible onto the surface tobe contacted, which may be, in the case of electrochemical cells, metalsurfaces (for examples electrodes or metal partition sheets) but alsosurfaces provided with a remarkably lower planar conductivity, (forexample gas diffusion electrodes made of carbon material). The planarlayers of interwoven wires are affected by the inconvenient, typical ofthe prior art, of a non sufficient deformability to grant an adequatecompression load for a sufficiently broad compression range. It istherefore preferable that the collector comprise internal layers ofwires having a permanent undulation according to a geometry easilyachieved through automatic working of the product. The best way to putthe invention into practice is providing at least two of these internallayers, sandwiched in order to have the more offset positioning of theundulation direction, for example the direction of the undulations ofadjacent layers may be about 90°. In this way, it is possible to obtainthat the internal undulated layer do not penetrate into each other andthis gives to the whole structure compression characteristics which aremuch more regular and gradual in terms of applied load with respect tothe compression thickness. In a further preferred embodiment, thevarious layers of the collectors are held together by a rigid perimetralframe which presents the further advantage of providing the object witha non deformable geometry with respect to the plane. In this way, thecollector may be easily applied into the complex arrangements made ofseveral cells, for example in conventional electrolysers or fuel cellstacks, made of filter-press arrangements of elementary cells, even whenan automated assembling system is foreseen to reduce considerably theproduction and maintenance costs. The above illustrated structurefurther offers the non negligible advantage of being open to the flow offluids with respect to traditional mats, which concurs to a betterreliability and operation efficiency of electrochemical processescarried out in the cells in which it is applied.

[0010]FIG. 1 shows an electrochemical cells containing the collector ofthe invention.

[0011]FIG. 2 shows a filter-press arrangement of electrochemical cellscontaining the collector of the invention.

[0012]FIG. 3 shows a preferred embodiment of the collector of theinvention.

[0013]FIG. 4 shows the load curves relating to the collector of theinvention compared to those of a prior art mattress.

[0014]FIG. 1 shows a generic electrochemical cell (1) divided by amembrane or diaphragm (2) and delimited by two conductive plates (3);the cell may generically be an electrolysis cell or a fuel cell or othertype of electrochemical reactor. The electrodes whereon the anodic andcathodic reactions take place are indicated by (4). The electrodes (4)may be any type of electrode generically known for electrochemicalapplications, for example metal sheets optionally activated byelectrocatalytic coatings, gas diffusion electrodes obtained on poroussurfaces such as carbon cloth or graphite, sinterized metals, etc. Forexample sake the perimetral gaskets (5) are also illustrated, but as itwill be obvious for an expert of the art, other hydraulic sealingsystems are likewise possible. For the types of electrodes described andfor other commonly used electrodes, it is not convenient to obtain theelectrical contact directly on the conductive plates (3). In fact thiswould require a too high and unpractical thickness as in many cases theporosity would be insufficient and, more importantly, the structures aresubstantially rigid and would require too high clamping pressures toobtain a good electrical contact, risking to damage irreversibly themembrane (2). The electric contact transmission between the plate (3)and the adjacent electrode (4) is therefore preferably effected by acompressible resilient material, preferably operating in an elasticmode. This material, in the case of FIG. 1, is the current collector (6)of the invention. However, it is evident to the expert in the art thatthis is only one of the different possibilities for using the collectorof the invention in an electrochemical cell and the same could beadvantageously used for example for contacting two metal platesbelonging to two adjacent cells in a monopolar or bipolar filter-presselectrolyser, or other similarly obvious applications.

[0015]FIG. 2 shows the collector of the invention used in similar cellsas those of FIG. 1 in a filter-press arrangement, in the specificbipolar case. The deformability characteristics of the collector of theinvention (6) and its adaptability to conform to the profile of thevarious metallic plates (3), for stacks which may reach even a hundredelementary cells (1), is particularly outstanding.

[0016]FIG. 3 shows a preferred embodiment of the multi-layered currentcollector (6), according to the invention. In this case the referencenumeral (7) indicates the two external layers, and (8) the two internallayers; it is quite evident how the current collector (6) may beproduced also with a different number of internal layers. The externallayers (7), obtained by interwoven metal wires, preferably with adiameter comprised between 0.1 and 0.35 millimeters, generally have aplanar profile. The external layers (8) are substantially the same asthe external ones, likewise made by interwoven metal wires (forsimplicity sake not shown in detail in the figure), apart from the factthat they are undulated, by means of a very simple mechanical working,in order to form a regular arrangement of protrusions (9) anddepressions (10), preferably regularly spaced apart. As will be seen inthe figure, preferably the direction of one undulation should be off-setwith respect to the underlying one; in the case of the two internallayers (8) of FIG. 3, the undulations are offset by 90°. In this manner,it is nearly completely avoided a reciprocal penetration of two internallayers (8). It has been found that arrangements of this type exhibitextremely gradual load curves as a function of the compression withrespect to prior art current collectors, so that a surprisingly broadrange is obtained, in terms of compression and thus of operatingthickness, whereby the applied mechanical load is sufficient to permit agood electrical contact without damaging the delicate components of thecell.

[0017]FIG. 4 shows the load curve (12) of a nickel collector obtained bytwo external mono-layers (7) made of a wire with a diameter of 0.27 mm,not undulated, and two internal double layers (8) made of a wire with adiameter of 0.16 mm, undulated with a pitch of 8.6 mm, put on top one ofthe other in order to off-set the undulations by 90° and avoidreciprocal penetrations. The four layers have been inserted in aperimetral frame in the form of a casing, not shown in the figures. Theoverall uncompressed thickness was about 10 millimeters. It can be seenfrom curve (12) that the useful compressed thickness range in theelastic mode, that is when the resulting load is comprised between 0.15and 0.40 kg/cm², varies between 3.6 and 5.4 millimeters, with acomprised between 46 and 64% with respect to the uncompressed thickness.It is a wide range, which easily complies with the tolerances of aconventional cell structure construction. The load curve (11) regards amattress, 6 millimeters thick, made by the same nickel wire, accordingto the teachings of U.S. Pat. No. 4,693,797: it is readily apparent thatthe useful operating range is extremely reduced, largely below 10% ofthe uncompressed thickness.

1. A substantially planar current collector comprising a sandwich ofcompressible and resilient layers consisting of metal wires, wherein itimparts a load between 0.15 and 0.40 kg/cm² in a compression range notlower than 10% of its uncompressed thickness.
 2. The collector of claim1, wherein the said compression range is comprised between 20 and 60% ofsaid uncompressed thickness.
 3. The collector of claim 1, wherein thesaid wires have a diameter comprised between 0.1 and 0.35 millimeters.4. The collector of claim 1 wherein it has an uncompressed thicknessbetween 5 and 15 millimeters.
 5. The collector of claim 1 wherein it ismade of a material selected from the group consisting of nickel,titanium and alloys thereof, optionally provided with a protectivecoating.
 6. The collector of claim 5 wherein the said protective coatingis silver or a noble metal.
 7. A current collector comprising a sandwichof compressible and resilient layers consisting of metal wires, whereinthe said layers of metal wires comprise two external planar layers andat least two internal undulated non-penetrated layers.
 8. The collectorof claim 7 wherein the said internal layers are undulated along twodirections off-set by about 90°.
 9. The collector of claim 7 wherein itimparts a load between 0.15 and 0.40 kg/cm² in a compression range notlower than 10% of its uncompressed thickness.
 10. The collector of claim9, wherein the said compression range is between 20 and 60% of saiduncompressed thickness.
 11. The collector of claim 9, wherein the saidwires have a diameter between 0.1 and 0.35 millimeters.
 12. Thecollector of claim 7 wherein it has an uncompressed thickness between 5and 15 millimeters.
 13. The collector of claim 7 wherein it is made of amaterial selected from the group consisting of nickel, titanium andalloys thereof, optionally provided with a protective coating.
 14. Thecollector of claim 13 wherein the said protective coating is silver or anoble metal.
 15. The collector of claim 7 wherein it comprises aperimetral frame supporting said sandwich of compressible and resilientlayers.
 16. An electrochemical cell comprising at least one currentcollector of claim
 1. 17. The cell of claim 16 wherein it is dividedinto at least two compartments by an ion exchange membrane or adiaphragm.
 18. The cell of claim 17 selected from the group consistingof fuel cells, chlor-alkali electrolysis cells, hydrochloric acidelectrolysis cells and neutral salt electrolysis cells.
 19. The cell ofclaim 16 wherein said current collector is in direct contact with a gasdiffusion electrode.
 20. The cell of claim 17 wherein said twocompartments are an anodic compartment and a cathodic compartment. 21.The cell of claim 20 wherein the said collector is made of pure titaniumor alloys thereof, optionally provided with a protective coating ofnoble metals or their oxides and said collector is placed in the anodiccompartment.
 22. The cell of claim 20 wherein the said collector is madeof pure nickel or alloys thereof, optionally provided with a protectivecoating of silver and said collector is placed in the cathodiccompartment.
 23. (cancelled)